The subject matter disclosed herein relates generally to touch sensors and touch sensor systems, and more particularly to projected capacitive touch sensors.
In a projected capacitive touch sensor, an outer surface may be provided over one or more layers having sense electrodes or sensors formed thereon. In contrast to common resistive touch sensors, the outer surface of a projected capacitive touch sensor may be a durable glass surface having high optical transparency for viewing images displayed by an underlying display device. The touch sensor may be positioned over a display device that displays graphical selections such as buttons and icons. When a user's finger touches the outer surface at a location corresponding to a desired selection displayed on the display device, the touch sensor system senses a change in capacitance associated with one or more of the electrodes. As used herein, a “projected capacitive” touch sensor is any capacitive touch sensor with a plurality of detection electrodes in the touch sensitive area, in contrast to a “surface capacitive” touch sensor that has a single detection electrode that covers the entire touch area.
Some projected capacitive touch sensors detect where a touch is located by measuring capacitance and then calculating (X,Y) coordinates. These detection algorithms may not yield accurate results in electrically noisy environments.
Each touch on the projected capacitive touch sensor is typically detected by at least two electrodes. The number of electrodes may vary depending on the size of the screen as well as the resolution desired.
For example, one type of a projected capacitive touch sensor system may have two electrode layers: a first electrode layer having parallel linear electrodes in a first direction and a second separate electrode layer having parallel linear electrodes in a direction perpendicular to the first direction, where the second electrode layer overlaps the first electrode layer. A virtue of such two electrode layer systems that have proven to be of interest to the marketplace is the ability to support two or more simultaneous touches as is used in two-finger zoom gestures. While being expensive to manufacture with its multiple electrode layers, this type of projected capacitive touch sensor system has the advantage of experiencing only modest coordinate distortions in the presence of electrical noise. The calculation of coordinates of a touch based on measured capacitance is susceptible to electrical noise. For instance, a 5% noise level may distort a coordinate measurement by about 5% of the width of a finger touch for this type of two-layer projected capacitive touch sensor. This level of distortion may be unacceptable for certain applications of the touch sensor.
Another type of a projected capacitive touch sensor system may have a “backgammon”-type electrode pattern configuration on a single layer containing two interleaved sets of generally triangular electrodes: one set (“set 1”) with triangles pointing in one direction (e.g., up) and the other set (“set 2”) with triangles pointing in the opposite direction (e.g., down), such as described in U.S. Pat. No. 6,297,811, which is incorporated herein by reference in its entirety. For such a backgammon-type system having a 3.5 inch diagonal measurement, the touch sensor may utilize close to fifty separate triangular-shaped electrodes on the single layer, and a seven-inch system may have more than one hundred electrodes. The single layer backgammon-type electrode configuration can provide multiple touch capability when pairs of touches excite disjoint sets of triangular-shaped electrodes, but has difficulty when the sets of excited electrodes from two simultaneous touches are not disjoint. For example if the triangular-shaped electrodes are aligned horizontally, detection of a pair of touches with similar vertical coordinates is problematic. Using a backgammon-type electrode configuration, the touch sensor may calculate two-dimensional coordinates after measuring capacitances from a single electrode layer, but unfortunately may be quite susceptible to electrical noise, which may negatively impact the determination of coordinates. For example, a 5% noise level may distort a coordinate (e.g. Y coordinate) measurement by 5% of the entire height of the touch area, which may be unacceptable for many touch applications.
With such concerns due to the low noise level requirements, the electronics required for these conventional projected capacitive touch sensor systems may drive the overall system production costs up, especially for larger touch sensor systems.
Accordingly, there is a need for low cost and higher noise-tolerant electrode touch sensor systems, such as projected capacitive touch systems, capable of detecting two or more simultaneous touches.